Silence of the clams

Environmentalists have long charged that dams and water diversions are killing the Colorado River and its delta (HCN, 7/3/00: A river resurrected: The Colorado River Delta gets a second chance). Now, scientists have quantified those accusations by counting clams. Their conclusion: The delta has lost 95 percent of its biological richness since Hoover Dam went up in the 1930s.

The researchers, led by Karl Flessa, a geoscientist at the University of Arizona, used clams as a stand-in for the delta's overall health. Clams are a staple food for many fish, mammals and migratory shorebirds, and they also leave behind proof of past abundance; in the delta, there are huge white beaches make up entirely of clam shells. The researchers did lab analyses of the clamshells to calculate historical numbers, and they used field surveys and satellite data to estimate the current clam population.

In the December issue of the journal Geology, the researchers published their results: Though some 6 billion bivalves, spread 50 to a square meter, flourished throughout the last 1,000 years, surveys last year found just three per meter.

"There has been a huge drop in the productivity of the delta," writes Flessa. The decline, he says, is "clearly" caused by the diversion of 90 percent of the river, or about 13.5 million acre-feet of water a year, to the fields and booming cities of the Sun Belt.

David Hogan of the Tucson-based Center for Biological Diversity says the new study could be useful to legal efforts to secure more water for the delta. Says Hogan, "You need numbers to negotiate with and show decline, and this gives us stark numbers."